FIELD OF THE INVENTION
This invention relates in general to seismic prospecting. In particular, it relates to seismic prospecting methods wherein seismic data having different spectral characteristics are combined.
To locate reflecting interfaces in the earth by seismic prospecting methods, seismic signals are transmitted through the earth, reflected by subterranean interfaces, and then detected and recorded. The time lapse between the transmission and detection of such a seismic signal gives the two way travel time of the seismic signal through the earth. The measured time lapse is then used to locate the interface.
Typically many subterranean interfaces are present in the earth, and a transmitted signal is reflected by a number of such interfaces to produce a number of reflected signals. The reflected signals from a single transmitted signal may be recorded as a seismic data trace. If the reflected signals do not superimpose significantly on the seismic data trace, the arrival times of the reflected signals may be readily determined to locate the subterranean interfaces. However, the interfaces are difficult to locate if many reflected signals are superimposed on each other.
If a vibratory seismic source is used, the seismic signal generated is of relatively long duration and the reflected signals from different interfaces typically superimpose upon one another. A typical signal transmitted by a vibratory seismic source consists of a sine wave sweeping over a frequency spectrum as a function of time. The similarity of the reflected signals to the transmitted signal is often masked because many arriving reflected signals are superimposed on each other. Thus, it is difficult to locate arrival times by visual observation of the seismic record. Correlation methods have been developed to help solve such problems.
The cross-correlation function of the transmitted signal and the received signal is a graph of the similarity between the two signal waveforms as a function of the time shift between them. The waveform of the received signal is obtained from a recording of the received signal. The receiver recording typically starts at the time when transmission of the seismic signal begins. In the correlation process, the instantaneous amplitudes of the received signal and of the transmitted signal are multiplied and the product summed over the duration of the transmitted wave form. The process is repeated with the transmitted signal progressively shifted in time relative to the received signal, and the summations are plotted against the time shifts to produce a cross-correlation curve. For a fuller exposition of the principles of correlation, see "Correlation Techniques--A Review" by N. A. Anstey in Geophysical Prospecting, Volume 12, No. 4 (1964), pages 355-382.
Correlation methods have been developed wherein several different portions of selected swept sine wave seismic signals are transmitted, successively in some methods and simultaneously in others. For example, U.S. Pat. No. 4,037,190, issued July 19, 1977 to Martin, discloses a method including the steps of successive generation of a number of vibrator sweeps having different beginning and final frequencies, digitization of each received seismic signal, correlation of each digitized received signal with its associated digitized driving signal ("sweep signal") to produce a number of correlograms each corresponding to a sweep signal, and thereafter the summing or "stacking" of the correlograms. The individual correlograms consist of approximately "zero phase" wavelets. Two problems are encountered with stacked vibrator data produced by this method. First the zero phase nature of the correlogram wavelets makes it difficult to relate the reflection arrival time (and polarity) determined therefrom to that determined from dynamite data. Also the zero phase nature of the correlogram wavelets makes it difficult to pick first arrivals.
Correlation methods of the type described above are also useful for combining or stacking seismic data generated by two or more seismic sources (which may or may not be vibratory seismic sources) each of which has different spectral characteristics.
Another conventional seismic prospecting method alleviates the two problems associated with the correlation methods described above, but results in data having poor resolution and further is not suitable for combining data from two or more seismic sources each of which has different spectral characteristics. Such method (which shall be denoted as the "deconvolved sweep" method) includes the steps of performing a vibrator sweep and recording a seismic data trace resulting therefrom, performing least-squares deconvolution on the sweep signal, and then correlating the data trace with the deconvolved sweep signal to produce an approximately minimum phase signal. However, the approximately minimum phase signal produced according to this deconvolved sweep method possesses a nearly rectangular frequency amplitude spectrum and is therefore quite "ringy". Such nearly rectangular spectra are associated with very cyclic pulses with large side-lobes, and thus with the concomitant poor resolution, just as sharp cutoff filters are ringy and hence result in poor resolution. If minimum phase signals produced by the deconvolved sweep method are added together, the resulting summed signal will not, in general, be minimum phase or even approximately minimum phase.
It has not been known, until the present invention, how to combine seismic data having different spectral characteristics, or how to combine data resulting from two or more seismic vibrator sweeps, in a manner which alleviates all of the above-noted problems associated with traditional seismic prospecting methods. In particular, it has not been known, until the present invention, how to employ correlation methods to combine two or more seismic data traces, each of which data traces consists of one or more wavelets, in such a manner that the sum of the individual correlated wavelets is approximately minimum phase and so that the sum of the individual correlated wavelets possesses a frequency amplitude spectrum which is the sum of the frequency amplitude spectrum of the individual correlated wavelets.